A standard Unix programming interface from The Open Group, which governs compliance. Formerly known as Spec 1170, and containing more than 1,100 API calls, products branded with the UNIX 95, UNIX 98 and UNIX 03 logos conform to Versions 1, 2 and 3 of the specification respectively. UNIX is a registered trademark of The Open Group and spelled with upper case letters. See Unix like and Unix.

UNIX 98
UNIX 98 is the branding for Version 2 of the specification. It adds support for real-time processing, threads, Y2K compliance and architecture neutrality. About half of the original APIs were derived from X/Open's XPG4 operating system. Also included are the X/Open Transport Interface (XTI), X and Motif interfaces, Unix sockets and curses. See UNIX 95 and UNIX 98.

UNIX 03
UNIX 03 is the branding for Version 3 of the specification. Introduced in late 2002, it makes the POSIX interface a mandatory requirement for certification (see POSIX). See UNIX 03 and Open Group.